"I tried the Duracell NiMH. They are white casing and rated at 8.4V. After a fresh charge they clocked in at 10.41V. The mini did not like this battery. Perhaps when it hit ~10 bps it would vent and need to be reset. Easily reproducable when you put the mode to ramping. No chops, just venting. Changing the battery back to an alkaline Duracell allowed the mini to function flawlessly again. - Yochinaman"

I ordered some "Tenergy" 9V rechargables off Amazon to try in my axe. They fit very well into the battery compartment. I have only gotten to play once with one in the gun due to weather and such so I can't really give an informed opinion yet.

Stay away from Energizer rechargables in general, they turn to garbage quickly. Sanyo Eneloop if possible, Tenergy as a second choice.

The issue with using that style of battery is that they slowly wear down with every charge, putting out less and less power. This will last a few times but soon you will start seeing issues with the marker.

I've had good lucky with lithium batteries when I had my mini, they tend to keep their voltage relatively level until they die unlike other batteries that will fade and cause the marker to malfunction when they wear down.